For a variety of applications, lasers must exhibit single longitudinal mode, narrlow lasing linewidth smooth wavelength tuning and power stability. These applications include fiber optic communications, high-resolution spectroscopy, optical recording and optical readout, microwave and millimeter wave generation. Semiconductor lasers, to be useful for these applications, must be capable of providing not only smooth wavelength tuning and operational stability, but also high-power, narrow-linewidth, and single-wavelength characteristics. The conventional standing-wave semiconductor lasers, however, do not satisfy these requirements. Because of their economy and reliability, however, it would be commercially attractive to use semiconductor lasers for these applications.
As local oscillators for fiber optic communications and high-resolution spectroscopy, lasers providing smooth wavelength tuning and narrow-linewidth, single-wavelength performance are desirable. See T. Okoshi, "Recent Advances in Coherent Optical Fiber Communication Systems," Journal of Lightwave Technology, Vol. LT-5 pp. 44-52 (1987). Narrow-linewidth and smooth wavelength tuning characteristics provide the capability for tuning the laser to the resonance transition. Also, fiber optic communications involving testing of optoelectronic devices need lasers that provide a wavelength tunable characteristics. See H. Sasada, "1.5 Micron DFB Semiconductor Laser Spectroscopy of HCN," J. Chemical Phys., Vol. 88, pp. 767-777 (1988).
Systems that employ optical recording and optical readout technology, use solid state lasers which emit visible light by a process known as second harmonic conversion. See W. J. Kozlovsky and W. Lenth, "Generation of 41 mW of Blue Radiation by Frequency Doubling of a GaAlAs Diode Laser, App. Phys. Lett., Vol 56, pp. 2291-2 (1990)". Second harmonic conversion is a desirable process, because of its potential for producing short wavelength light. Short wavelength light has the ability to provide improved information storage capacity for optical recording relative to longer wavelength light. Second harmonic conversion achieves a shorter wavelength by optically pumping a solid state material (e.g., Potassium Niobate (KNbO.sub.3)) with a high-powered infrared semiconductor laser. The improved information storage capacity of light produced by second harmonic conversion has commercial applications and may make possible more advanced research in optical recording and optical readout.
Another use for wavelength-tunable lasers is in microwave and millimeter wave signal generation. See L. Goldberg, A. M. Yurek, H. F. Taylor and J. F. Weller, 35 GHz Microwave Signal Generation with an Injection Locked Laser Diode" Electron Lett., 21, pp. 714-815, (1985). By mixing lasing light from two laser devices, a third microwave or millimeter wave frequency signal may be generated. Generation of the radio frequency signal in this way, however, requires coherent laser radiation with high stability and wavelength tuning smoothness for quality high frequency signal generation. Known systems for use in microwave and millimeter wave signal generation have substantial room for improvement in both smooth wavelength tuning and performance stability.
A further application for lasers that provide smooth wavelength tuning characteristics is in fiber sensors. If a laser provides a minimum nonlinear gain, it may be useful for picosecond pulse generation applications in fiber sensors. A use for this type of device may be, for example, as a component for high-speed sampling oscilloscopes.
Semiconductor lasers have heretofore been unable to meet the requirements of the above applications. Known external-cavity semiconductor lasers are standing-wave devices in which coherent optical radiation travels in two directions. These devices exhibit a phenomena known as "mode hopping" or "wavelength hopping" that is undesirable for the above laser applications. Two factors contribute to the presence of mode hopping.
First, a two-cavity situation occurs within a semiconductor laser because of non-zero facet reflectivities of the semiconductor material. Within the laser cavity, light is reflected between a laser facet and the mirrored surface of the external cavity. The semiconductor material itself, however, has inherent non-zero reflectivities and forms a second cavity within which the light reflects. This two-cavity situation makes it difficult to align the external-cavity standing-wave laser system for a variety of laser frequencies. Thus, when tuning occurs, mode hopping results and laser performance degrades due to a phase mismatch between the cavity of the semiconductor material facets and the cavity of the mirrored surface of the laser external cavity.
The other characteristic of standing-wave semiconductor laser systems that causes mode hopping and other performance instabilities is the presence of an intrinsic dielectric grating. The dielectric grating occurs because of the standing-wave. The result is a dielectric spatial perturbation that affects the output frequency. Because of the dielectric grating, the standing-wave semiconductor laser is more likely to exhibit optical instability. Moreover, the index change of the semiconductor material due to temperature and current fluctuation causes performance instability. This is because the semiconductor cavity significantly affects the operation of the device.
Consequently, there is a need for a wavelength-tunable semiconductor laser for a variety of applications including fiber optic communications, high-resolution spectroscopy, optical recording and optical readout, microwave and millimeter wave generation, and picosecond pulse generation, for fiber sensors.
There is a need for a laser that provides the above attributes in an economical package that is both simple to use and highly reliable.
Moreover, there is a need for an improved semiconductor laser that provides smooth wavelength tuning and narrow-linewidth, single-wavelength spectral characteristics.
Further, there is a need for a laser that avoids mode hopping during wavelength tuning and minimizes nonlinear gain to produce a more stable laser output during tuning and under steady state operations.